Change Object Relative Position to Origin
There are several ways of changing an object's origin to a specific place.


Place the 3D cursor where you want the origin of the object to be.  This can be done by either modifying it's position directly under the 3D Cursor roliout of the N toolbar in the 3D view, or by selecting something and pressing Shift+S > 3D Cursor to Selected.   Then select the object and hit CtrlShiftAlt+C > Origin to 3D Cursor.
You can set an object's origin to the world origin by applying its location with Ctrl+A > Apply Location.